Among the various video display systems available in the art, an optical projection system is known to be capable of providing high quality displays in a large scale. In such an optical projection system, light from a lamp is uniformly illuminated onto an array of, e.g., M.times.N, thin film actuated mirrors, wherein each of the mirrors is coupled with each of the actuators. The actuators may be made of an electrodisplacive material such as a piezoelectric or an electrostrictive material which deforms in response to an electric field applied thereto.
The reflected light beam from each of the mirrors is incident upon an aperture of, e.g., an optical baffle. By applying an electrical signal to each of the actuators, the relative position of each of the mirrors to the incident light beam is altered, thereby causing a deviation in the optical path of the reflected beam from each of the mirrors. As the optical path of each of the reflected beams is varied, the amount of light reflected from each of the mirrors which passes through the aperture is changed, thereby modulating the intensity of the beam. The modulated beams through the aperture are transmitted onto a projection screen via an appropriate optical device such as a projection lens, to thereby display an image thereon.
In FIGS. 1A to 1G, there are illustrated manufacturing steps involved in preparing an array 10 of M.times.N thin film actuated mirrors 11, wherein M and N are integers, disclosed in a copending commonly owned application, U.S. Ser. No. 08/430,628, entitled "THIN FILM ACTUATED MIRROR ARRAY".
The process for manufacturing the array 10 begins with the preparation of an active matrix 20 comprising a substrate 22, an array of M.times.N transistors(not shown) and an array of M.times.N connecting terminals 24.
In a subsequent step, there is formed on top of the active matrix 20 a thin film sacrificial layer 40 by using a sputtering or an evaporation method if the thin film sacrificial layer 40 is made of a metal, a chemical vapor deposition(CVD) or a spin coating method if the thin film sacrificial layer 40 is made of a phosphor-silicate glass(PSG), or a CVD method if the thin film sacrificial layer 40 is made of a poly-Si.
Thereafter, there is formed a supporting layer 15 including an array of M.times.N supporting members 30 surrounded by the thin film sacrificial layer 40, wherein the supporting layer 15 is formed by: creating an array of M.times.N empty slots(not shown) in the thin film sacrificial layer 40 by using a photolithography method, each of the empty slots being located around the connecting terminals 24; and forming a supporting member 30 in each of the empty slots by using a sputtering or a CVD method, as shown in FIG. 1A. The supporting members 30 are made of an insulating material.
In a following step, an elastic layer 70 made of the same insulating material as the supporting members 30 is formed on top of the supporting layer 15 by using a Sol-Gel, a sputtering or a CVD method.
Subsequently, a conduit 35 made of a metal is formed in each of the supporting members 30 by: first creating an array of M.times.N holes(not shown), each of the holes extending from top of the elastic layer 70 to top of the connecting terminals 24, by using an etching method; and filling therein with the metal to thereby form the conduit 35, as shown in FIG. lB.
In a next step, a second thin film layer 60 made of an electrically conducting material is formed on top of the elastic layer 70 including the conduits 35 by using a sputtering method. The second thin film layer 60 is electrically connected to the transistors through the conduits 35 formed in the supporting members 30.
Then, a thin film electrodisplacive layer 80 made of a piezoelectric material, e.g., lead zirconium titanate(PZT), is formed on top of the second thin film layer 60 by using a Sol-Gel, a sputtering or a CVD method, as shown in FIG. 1C.
In an ensuing step, the thin film electrodisplacive layer 80, the second thin film layer 60 and the elastic layer 70 are patterned into an array of M.times.N thin film electrodisplacive members 85, an array of M.times.N second thin film electrodes 65 and an array of M.times.N elastic members 75 by using a photolithography or a laser trimming method until the supporting layer 15 is exposed, as shown in FIG. 1D. Each of the second thin film electrodes 65 is connected electrically to a corresponding transistor through the conduit 35 formed in each of the supporting members 30 and functions as a signal electrode in each of the thin film actuated mirrors 11.
Next, each of the thin film electrodisplacive members 85 is heat treated to allow a phase transition to take place to thereby form an array of M.times.N heat treated structures(not shown). Since each of the thin film electrodisplacive members 85 is sufficiently thin, there is no need to pole it in case it is made of a piezoelectric material: for it can be poled with the electric signal applied during the operation of the thin film actuated mirrors 11.
After the above step, an array of M.times.N first thin film electrodes 55 made of an electrically conducting and light reflecting material, e.g., aluminum(Al) or silver(Ag), is formed on top of the thin film electrodisplacive members 85 in the array of M.times.N heat treated structures by first forming a layer 50, made of the electrically conducting and light reflecting material, completely covering top of the array of M.times.N heat treated structures, including the exposed supporting layer 15, using a sputtering method, as shown in FIG. 1E, and then selectively removing the layer 50, using an etching method, resulting in an array 90 of M.times.N actuated mirror structures 95, wherein each of the actuated mirror structures 95 includes a top surface and four side surfaces, as shown in FIG. 1F. Each of the first thin film electrodes 55 functions as a mirror as well as a bias electrode in each of the thin film actuated mirrors 11.
The preceeding step is then followed by completely covering the top surface and the four side surfaces in each of the actuated mirror structures 95 with a thin film protection layer(not shown).
The thin film sacrificial layer 40 in the supporting layer 15 is then removed by using an etching method. Finally, the thin film protection layer is removed by using an etching method to thereby form the array 10 of M.times.N thin film actuated mirrors 11, as shown in FIG. 1G.
There are certain deficiencies associated with the above described array 10 of thin film actuated mirrors 11 and the method for the manufacture thereof. During the removal of the thin film protection layer, an etchant used therein may chemically attack the first thin film electrode 55, which also functions as a mirror, in each of the thin film actuated mirrors 11, which may adversely affect the optical efficiency of the array 10 of thin film actuated mirrors 11. In addition, the first thin film electrode 55 may become oxidized, especially when the first thin film electrode 55 is made of Ag, further reducing the reflectivity thereof.